The clutch throw out arm sits on the outside of the 'wok' and transfers the motion of the slave cylinder to the plunger inside the wok.
These arms work hard and wear or even break the ball off the end. Worn arms can contribute to clutch shudder and difficulty in adjusting the clutch.
This is the later thicker style, for all pre verto (before 1981) setups. A good item to replace when you are curing clutch problems of poor release or shudder.